Classification of Rocks. — Spurr. 223 
sible," and the further suggestion that it would be desirable if 
the true porphyric equivalents of the granitoid structure should 
have nearly the same appellations. 
Zirkel* bases his classification as tabulated mainly on (i) 
mineral composition, (2) age, and (3) structure. His age di- 
visions are, like Rosenbusch's alid Michel-Levy's, only two, 
and apply only to his porphyritic and glassy rocks, which he 
distinguished as pre-Tertiary or Tertiary and post-Tertiary. 
Broggert publishes a classification of the feldspar rocks 
which is based (i) on mineralogy, (2) chemistry, (3) structure. 
Under mineralogy, only the feldspars are considered in this 
classification, and under chemistry only the percentage of 
silica. Under the head of structure Brogger has three di- 
visions, corresponding essentially to Michel-Levy's granitoid, 
porphyric, and trachytoid. 
Teall's;]; classification is based (i) on mineralogy, (2) on 
structure, and (3) on chemistry. Under structure he has only 
two major divisions, granitic and trachytic, with subdivisions 
under the latter. Chemistry is used only in so far that the 
relative percentage of silica is followed in the arrangement of 
the groups. 
Kemp, in his Handbook of Rocks, § also sketches a rock 
classification based (i) on mineralogy, (2) on structure, and (3) 
on chemistry. His structures (omitting the glassy and the 
fragmental, the latter of which does not belong to igneous 
rocks) are essentially two, — granitoid ,andfelsitic and porphy- 
ritic, corresponding to the divisions of Teall. As in Teall's 
classification, the chemical element is considered only in the 
arrangement of mineralogic groups, following as nearly as 
possible the order of their relative content of silica. 
No attempt has been made to make the enumeration of pro- 
posed classifications exhaustive, but only the more important 
examples, illustrating dift'erent views, have been cited. 
The classification adopted in the present report is based on 
(1) mineralogy, (2) structure, (3) chemical relations. In the se- 
* Lehrbuch der Petrographie, Leipzig, 1893. P- 834. 
tEruptionsfolge der Triadischen Eruptixgesteine bei Predaz/.o in 
Siidtyrol, Kristiania, 1895, p. 60. 
^British Petrograjihy, Birmingham, 1886. p. 70. 
§New York, 1896, p. 18. . 
